1. Field of the Invention
The present invention relates to an optical disk, an optical disk recording/reproduction apparatus, and a method for evaluating optical data of the optical disk.
2. Related Background Art
Optical disks are classified into two types: read-only optical disks on which data are recorded in advance and optical disks on which data can be recorded. To record data on the read-only optical disk, generally, an exposure process called a mastering process is performed to form embossments (physically raised and recessed conditions) on the optical disk. Then, to record data on the recordable optical disk, the optical disk is irradiated with a focused laser beam to change a specific physical characteristic of a recording film.
Conventionally, when evaluating the quality of data or signals recorded on an optical disk, a measurement is generally made of the jitter characteristic of a reproduction signal that is obtained by irradiating with a laser beam and that is reflected by the optical disk. As is shown in FIG. 1, the jitter characteristic is usually represented as the timetransient fluctuation of an edge position that is obtained by slicing a reproduction signal at a specific reference potential.
However, as is apparent from FIG. 1, when the recording density is increased, and when a mark length is small, the signal amplitude is reduced so it does not extend across the slice level used for detecting the edge position. Therefore, performing the jitter measurement is difficult. Further, as is shown in FIG. 1, when the recording density is further increased to improve the recording capacity, the effect produced by intersymbol interference is increased and causes the signal amplitude to be reduced so that the signal amplitude does not extend across the slice level used for the detection of the edge position. As a result, performing the jitter measurement is difficult.
Conventionally, to reduce the intersymbol interference, a method has been employed that uses an equalizer to filter a reproduced waveform. However, while restricting the intersymbol interference, the equalizer generally increases the noise component. Thus, when the recording density is very high, it is difficult for the original recorded data to be obtained by decoding the reproduced signal.
As a method for accurately decoding data when the recording density is very high, there is a well known signal detection method called the PRML (Partial-Response Maximum-Likelihood) method. According to this method, a reproduced waveform is equalized (PR equalized) to provide a waveform that includes intersymbol interference for suppressing the noise component, and the data are identified by employing a method called Viterbi decoding (ML). In this instance, the PR equalization is defined by the amplitude for each data cycle (clock). For example, for PR(abc), the amplitude at time 0 is a, the amplitude at time T is b, the amplitude at time 2T is c and the amplitude at all other times is 0. The total number of components having an amplitude other than 0 is called a constraint length.
According to the PRML, instead of detecting the edge position to decode data, the value obtained by sampling a reproduced waveform each clock cycle is employed to obtain data through the Viterbi decoding. Therefore, it is difficult to estimate the detection function of the PRML based only on the time-transient fluctuation data for the edge position.